A newly-identified link between pandemic flu and the weather phenomenon known as La Niña, may one day permit advance warnings of severe influenza outbreaks.

Most of the time, influenza is a temporary annoyance. But every so often a super flu bug comes along, killing millions and sickening many more.

Jeffrey Shaman, of the Columbia University School of Public Health, notes there were four documented flu pandemics in the past century.

"When we look at those four events, we see that all four of them began directly after a La Niña event in the Pacific," he says.

La Niña is a periodic cooling of Pacific ocean waters that triggers changes in global weather patterns. Among other things, that altered weather disrupts bird migrations.

Birds can carry flu virus, and when their migratory patterns change, they can come into contact with other avian species they don't normally meet - birds which might carry a different strain of flu virus.

In the process, the viruses’ genetic material can get intermingled to create new influenza strains - in a process known as reassortment.

"And it's this reassortment, this creation of new sub-types that takes place - and we think it's in the bird population - that generates, potentially, these pandemic strains that can infect humans and to which most of the world's population will be susceptible," Shaman says.

La Niña events happen every few years, and most are not followed by a pandemic. But because the risk of a pandemic appears to increase after a La Niña, the next step for researchers is to get a better understanding of how birds and the flu viruses they carry are affected.

One result, Shaman says, may be the ability to improve prediction of an influenza pandemic.

"That's the thing that's exciting about it," he says. "I mean, it offers this sort of tantalizing possibility that you can say, we have a La Niña coming, we need to make these preparations because we know there's an increased likelihood that a pandemic flu strain could arise and infect humans."